The present invention relates to a donor sheet for pressure-sensitive image recording, and more particularly to a donor sheet for pressure-sensitive image recording giving record images of an increased density.
Hitherto, a carbon paper has been used as a pressure-sensitive image recording material, but they have exposed colored materials, so that it is easy to color and stain fingers and other articles.
Recently, however, a no carbon required paper (another name, carbonless paper) having a colorless appearance is being mainly used in which an electron-donative colorless dye and an electron-receptive developer are used in combination an image-forming components (for example, Japanese patent publication No. 37451/1971).
This colorless no carbon required paper is combination of a donor sheet and a receptor sheet, the former being usually produced by dissolving one of the image-forming components in a solvent, sealing the resulting solution into microcapsules and coating the microcapsules onto a supporting sheet, and the latter having a coating layer of the other image-forming component. When the donor sheet is laid upon the receptor sheet, and pressure is applied to the former sheet, the microcapsules at the pressure-applied portions are broken. Thus, the solution in the broken microcapsules transfers from the donor sheet to the receptor sheet and reacts with the developer to form a visible image on the receptor sheet.
Colored dyes or pigments may be used as the image-forming component (for example, Japanese Patent Application Kokai No. 39844/1987). In this case, the solution or dispersion of a colored dye or pigment is sealed into microcapsules and coated onto a supporting sheet to prepare a donor sheet. When this donor sheet is laid upon a receptor sheet which needs no developer, and then pressure is applied to it, the colored dye or pigment is transferred from the donor sheet to the receptor sheet to color the receptor sheet.
In these conventionally used techniques, the image density obtained depends first of all upon the amount of the image-forming component transferred from the donor sheet to the receptor sheet.
That is, the above no carbon required paper is designed so that the image-forming component for the receptor sheet is usually incorporated in a stoichiometric excess and reacted with the other image-forming component transferred from the donor sheet. Consequently, the image density is determined by the transfer amount.
In a case where transfer of colored dyes or pigments is utilized, it is obvious that the image intensity is determined by the transfer amount.
Hitherto, it is known that a rate at which the image-forming component sealed into the microcapsules of the donor sheet is transferred to the receptor sheet after released by rupture of the microcapsules by pressure, i.e. a transfer rate, is fairly smaller than a rate at which the released component remains in the donor sheet, and therefore that the image-forming component is not utilized in good efficiency for image forming (for example, Japanese patent publication No. 14037/1989).
Consequently, in pressure-sensitive image-recording materials such as no carbon required paper, in order to obtain a sufficiently high image density for practical use, the amount of the image-forming component to be incorporated in the donor sheet is fixed at a high level in view of the transfer rate being low. In short, this means that the image-forming component is not utilized effectively.
If the rate of transfer by applied pressure of the image-forming component to be incorporated in the donor sheet is more improved than now, the image density also improves the more, and besides the amount of the image-forming component necessary to obtain a certain image density can be reduced, which has a great significance in terms of industrial techniques.
The pressure-sensitive image-recording material will be explained here with reference to the no carbon required paper, which is a typical example of the material. Usually, the no carbon required paper comprises a top sheet (i.e. a donor sheet) and an under sheet (i.e. a receptor sheet), the former sheet having on one side a coating layer of microcapsules containing one image-forming component in solution in a solvent and the latter sheet having on one side a coating layer of the other image-forming component. The image-forming component to be incorporated in the top sheet is, in many cases, electron-donative colorless dye such as Crystal Violet Lactone, Benzoyl Leucomethylene Blue, Malachite Green Lactone, Rhodamine Anilinolactam or 3-diethylamino-6-methyl-7-anilinofluoran, but it may be an electron-receptive developer such as phenol resin or the zinc salt of salicylic acid derivatives. Further, said image-forming component may be one component of an oxidation-reduction color-development system or chelate color-development system comprising combination of a metallic compound and a ligand. The other image-forming component includes electron-receptive developers such as acid clay, activated clay, various phenol resins, the polyvalent metallic salt of salicylic acid derivatives, etc. When the both sheets are laid one upon another so that the respective coating layers face with each other, and then pressure is applied thereto by writing or impact printers, microcapsules at the pressurized portion on the top sheet are broken, and the solution of the component contained in the microcapsules is released. Some percentage of the released solution is transferred to the under sheet to develop a color thereon in the form of visible images including letters, symbols, figures, etc. At that time, if middle sheets having at one side a coating layer of microcapsules containing one image-forming component and at the other side a coating layer of the other image-forming component, are used insert between the top sheet and under sheet, many pieces of the record image are obtained.
As another example of the pressure-sensitive image-recording material, there is known a photo- and pressure-sensitive image-recording material. This material also comprises a donor sheet and a receptor sheet, the former sheet having on one side a coating layer of microcapsules containing a solution or dispersion of a colorless dye such as disclosed in Japanese patent application Kokai No. 88739/1983 or a colored dye or pigment such as disclosed in Japanese patent application Kokai No. 39844/1987 in a photocurable liquid prepolymer. When this donor sheet is exposed to light through an image pattern to cure the microcapsules at the exposed portion, and then pressure is applied to the whole surface of the donor sheet laid upon the receptor sheet, the microcapsules only at the non-exposed portion are broken and the contents of the microcapsules are released. Some percentage of the released contents is transferred to the receptor sheet to form an image. The transfer rate at this time also is not large, and a large proportion of the image-forming component is in vain without taking part in the image formation.
There are few literatures describing a method for improving the rate of transfer of the image-forming component from the donor sheet, but descriptions on the method are seen, for example, in Japanese patent application Kokai No. 43895/1982 and No. 182981/1986. The former patent discloses to simply incorporate a wax in the coating layer of the donor sheet. The method of the former patent is accompanied by a practically serious defect that since the coating layer becomes hydrophobic and water-repellent by incorporating the wax, the water-based glue (adhesive) cannot be used, and therefore that adhesion with a edge-padding glue, i.e. edge padding becomes impossible.
In the method of the latter patent, microcapsules containing as a core substance a liquid representative of which is an aliphatic hydrocarbon solvent alone are pre-coated onto the supporting sheet, and then microcapsules containing a colorless dye (coloring agent) are coated onto the above pre-coating layer. In this case the total coating amount of the microcapsules is about twice as large as that of the usual case, so that the total coating amount of the solvent also is about twice. Consequently, images obtained by application of pressure, particularly thin lines of letters and diagrams blur thick, and a record of letters of complicated strokes gives only copied letters which are obscure in the stroke, difficult to read and so poor in practical value.